a) Field of the Invention
This invention relates to a liquid crystal display with improved contrast ratio, switching performance, reflectivity at the Dmin state and structural integrity, and methods for its manufacture.
b) Background
A polymer dispersed liquid crystal (PDLC) display usually comprises two transparent plates with electrodes placed opposing each other, separated by using spacers. A thin film of PDLC is enclosed between the two plates. The PDLC film may be up to 200 microns thick, but usually having a thickness of between 2 microns and 50 microns. The liquid crystal filled cell is hermetically sealed in order to eliminate oxygen and moisture, both of which may chemically attack the liquid crystals. A thorough review of the PDLC technologies can be found in “Liquid Crystal Dispersions” by P. S. Drzaic (1995).
A PDLC typically consists of micron-size droplets of low-molecular-weight nematic liquid crystals dispersed in a polymer binder. The nematic droplets strongly scatter light and the material has a white opaque or translucent appearance (“off” state). When a voltage difference is imposed between the two electrodes (“on” state”), the electric field aligns the droplets such that the ordinary refractive index of the liquid crystals nearly matches that of the isotropic polymer matrix, substantially reducing the scattering power of the droplets, thus allowing light to transmit through. As a result, the cells appear clear or transparent in the “on” state and opaque in the “off” state.
In a guest-host PDLC display, a dye, particularly a pleochroic or dichroic dye, is added as a guest to the liquid crystals to produce a high color contrast display. For example, the dye molecules may have a property to orientate themselves parallel to the liquid crystal molecules. Therefore if a dichroic dye having a bar-shaped structure is added to the liquid crystals, the direction of the dye molecules also changes if the molecular direction of the liquid crystals is changed by applying an electric field on the opposing electrodes. Because the dyes are made colored or do not depend on the orientation direction, it is possible for them to switch between a colored state (“off” state) and a colorless state (“on” state) by applying a voltage on the two electrodes. The use of dichroic or pleochroic dyes in guest-host PDLC displays to improve the contrast ratio is known in the art.
A PDLC display may be transmissive and/or reflective. A transmissive PDLC display has an internal illumination source. Imposing a voltage on the two electrodes allows light to pass through the liquid crystal filled cells. A typical example of a transmissive PDLC display is a PDLC overhead projector. Reflective PDLC displays typically contain a reflective black or colored filter which becomes visible in the transparent state. Reflective PDLC displays may be found in PDA (personal digital assistant) devices. Transmissive and reflective PDLC displays are particularly attractive because polarizers are eliminated. Polarizers substantially reduce light and decrease brightness of both direct view and projection displays. The absence of polarizers also gives a better viewing angle.
The PDLC displays prepared by processes previously used have many shortcomings. For example, the polymer dispersed liquid crystals typically have droplets of a very broad particle size distribution, which results in significant hysteresis, higher operation voltage, poor contrast ratio, undesirable red bleedthrough and low level of multiplexing. However, the hysteresis of PDLC films must be low to show reproducible gray scales, and low voltage operation and high contrast ratio of the device are essential for most PDA applications. Monodispersed liquid crystal particles in the micron size range have been disclosed in U.S. Pat. No. 5,835,174, (Clikeman, et al.), U.S. Pat. No. 5,976,405 (Clikeman, et al.) and U.S. Pat. No. 6,037,058 (Clikeman, et al.) to reduce the hysteresis and operation voltage and to improve the level of multiplexity. The contrast ratio of the PDLC device prepared from the monodispersed particles remains low for most applications. To improve the contrast ratio without trade-off in the thickness of the PDLC film and operation voltage, guest dyes, preferably pleochroic dyes or dichroic dyes, are needed. However, the processes do not allow for the precise enclosure of a high concentration of guest dyes in the liquid crystal phase during manufacture, such that only a low concentration of dyes may be encapsulated in the monodispersed polymer particles. Some guest dyes may be left outside of the particles, thereby resulting in an increase in Dmin (the minimum optical density of the background) and a lower contrast ratio. In all cases, the process previously used involves a costly process to separate and purify particles from an aqueous phase followed by a process to re-disperse the particles in an organic binder.
An improved liquid crystal display was disclosed in U.S. Pat. No. 6,795,138 (corresponding to WO02/56097), the content of which is incorporated herein by reference in its entirety. The improved liquid crystal display comprises isolated cells formed from microcups of well-defined shape, size and aspect ratio and filled with a liquid crystal composition optionally comprising guest dye(s). The filled cells are individually sealed with a polymeric sealing layer, preferably formed from a composition comprising a material selected from the group consisting of thermoplastics, thermoplastic elastomers, thermosets and precursors thereof.
The microcup structure enables a format flexible and efficient roll-to-roll continuous manufacturing process for the preparation of liquid crystal displays. The displays can be prepared on a continuous web of a conductor film (such as ITO/PET) by, for example, (1) coating a radiation curable composition onto a conductor film, (2) forming the microcup structure by, for example, a microembossing or photolithographic method, (3) filling a liquid crystal composition into the microcups and sealing the filled microcups, (4) laminating the sealed microcups with another conductor film, and (5) cutting the display to a desirable size or format for assembling.
One advantage of this type of liquid crystal displays is that the liquid crystals are enclosed in the microcups. In one embodiment, the microcups may be of uniform size and shape. This significantly reduces the hysteresis and the reorientation field strength. In addition, before being filled with a liquid crystal composition, the microcups may be surface treated or modified to alter the surface properties for optimum contrast ratio and response rate.
The other advantage of this type of liquid crystal displays is that the microcup wall is in fact a built-in spacer to keep the top and bottom substrates apart at a fixed distance. The mechanical properties and structural integrity of microcup-based displays are significantly better than traditional dispersed liquid crystal displays. Moreover, the microcups may be sequentially filled with liquid crystal compositions of different specific properties such as colors and switching rates, and isolated by a sealing layer. Unlike the Clikeman's process of preparing monodispersed liquid crystal particles, guest dyes may be incorporated easily into a microcup-based liquid crystal display without involving time-consuming or costly procedures.
In traditional dispersed liquid crystal displays, the formulation window of compositions of both the liquid crystal and polymer phases is quite limited. In contrast, the two phases may be optimized independently in the microcup-based liquid crystal displays. Moreover, the microcups may be surface-treated, if necessary, to modify the interaction between the liquid crystals and the microcup walls to achieve optimal electro-optical response for various applications.
The whole content of each document referred to in this application is incorporated by reference into this application in its entirety.